The present invention is directed to methods for allocating and scheduling a forward and a reverse traffic channel and rescue channel resources for a code division multiple access (CDMA) communications system. More specifically, but without limitation thereto, the present invention is directed to synchronizing measurement of a location of a mobile station and CDMA communications service.
The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these satellites as reference points to calculate positions to an accuracy from a few meters up to less than a centimeter. GPS receivers have been miniaturized to just a few integrated circuits and may be advantageously combined with cellular telephone components to make the technology accessible to virtually everyone.
Basically, GPS is the geometric triangulation of a GPS receiver from at least three of the constellation of GPS satellites. The triangulation is performed by measuring the distance between the GPS receiver and each satellite using the travel time of radio signals. The accuracy of measurement of the travel time of the radio signals limits the accuracy of the triangulation. For example, at the speed of light, a measurement error of only a millionth of a second results in an error of three hundred meters. In addition to distance, the exact position of the satellites must be known. Also, the delays the radio signal experiences as it travels through the atmosphere must be taken into account, that is, the speed of light varies as the radio signal travels through the ionosphere and the atmosphere.
As an example of triangulation, suppose the travel time of the radio signal to a first satellite results in a distance of 22,000 kilometers. The location of the GPS receiver is then limited to a point on the surface of a sphere having a radius of 22,000 kilometers. Next, the distance to a second satellite is measured at 24,000 kilometers. The location of the GPS receiver is therefore on the surface a sphere having a radius of 24,000 kilometers from the second satellite. The location of the GPS receiver is now limited to a point on a circle where the two spheres intersect. If the measurement of the radio signal to a third satellite results in a distance of 26,000 kilometers, the position of the GPS receiver is narrowed down to the two points where the sphere of the third satellite having a 26,000 kilometer radius cuts through the circle that defines the intersection of the first two spheres, which occurs at two points on the circle. From three satellites, the position of the GPS receiver may be narrowed down to one of these two points. To decide which point is the true position, a fourth measurement may be made of the distance from a fourth satellite to the GPS receiver. Usually, however, one of the two points is obviously incorrect (either too far from the Earth or else moving at an impossible velocity) and may be rejected by logic without the necessity of making a fourth measurement. A fourth measurement is useful for another reason that will become apparent later.
Each distance used for the triangulation is calculated from measuring the travel time of a radio signal from the satellite to the GPS receiver. If the satellite is directly overhead, the travel time would be about 0.06 seconds. To achieve an accuracy of one meter from a distance of 20,000 kilometers, the measurement must have a precision of about one in 20 million, which requires eight significant digits. In other words, a travel time of 0.06 seconds must be distinguishable from a travel time of 0.059999999 seconds.
To achieve the required precision, the radio signal is modulated by a pseudo-random code. Each satellite has its own unique pseudo-random code to ensure that the GPS receiver does not confuse the signal from one satellite with that of another, so all the satellites can use the same frequency without jamming each other. The pseudo-random code also make possible the use of information theory to increase the signal-to-noise ratio of the GPS signal, which is why GPS receivers do not require large satellite dishes to receive the GPS signals.
The GPS receiver has a copy of the pseudo-random code pattern used by each satellite. The starting point of the copy is synchronized to universal time by a reference clock in the GPS receiver, and the copy is compared to the demodulated radio signal from the satellite at different delay times until the comparison of the delayed copy with the satellite signal reaches a peak value. The delay time at which the delayed copy matches the satellite signal is identical to the travel time.
The GPS satellites have extremely precise atomic clocks on board, however, both the satellite and the GPS receiver must precisely synchronize the pseudo-random code to accurately measure the travel time. This synchronization may be performed by measuring the distance to a fourth satellite. If the GPS receiver reference clock were perfectly identical to the satellite atomic clock, then all the satellite ranges would intersect at a single point, that is, the position of the GPS receiver. Realistically, however, the clocks are not identical, and the fourth measurement will not intersect with the first three measurements. Because any offset in the reference clock from universal time affects all of the measurements equally, the GPS receiver looks for a common time correction that it can apply to all four timing measurements that would result in their intersecting at a single point, which gives the position of the GPS receiver to the desired accuracy, and the correction is used to synchronize the GPS receiver's clock with universal time kept by the atomic clocks in the satellites.
The pseudo-random code provides a precise delay measurement, and the extra distance measurement synchronizes the reference clock in the GPS receiver to universal time kept by the satellite clock, but for referencing the triangulation to world coordinates, the exact position of the satellites in space must be determined. The high satellite altitude of 22,000 kilometers results in an orbit that may be described by very simple mathematics. Each GPS satellite has a very precise orbit according to the GPS master plan. On the ground, all GPS receivers have an almanac programmed into their computers that tells them where each satellite is at each moment. Also, the GPS satellites are constantly monitored by the Department of Defense by precise radar to check each satellite's exact altitude, position and speed. Any errors from the predicted orbit are called ephemeris errors because they affect the satellite's orbit, or ephemeris. These errors are caused by the gravitational attraction of the moon and the sun and by the pressure of solar radiation on the satellites. The ephemeris errors are usually very slight but must be taken into account to achieve a high accuracy. Once the satellite's exact position has been measured, the ephemeris information is transmitted to the satellite itself. The satellite then includes this new corrected position information in the pseudo-random code signals that it broadcasts.
As described above, the distance from the GPS receiver to a satellite is calculated by multiplying the travel time of the radio signal by the speed of light. However, the speed of light is only constant in a vacuum. As a GPS signal passes through charged particles of the ionosphere and through water vapor in the troposphere, the signal travels slower than in a vacuum, and this creates an error in the measurement. One way to minimize this speed error is to predict what a typical speed of light might be on a typical day, however, atmospheric conditions are rarely exactly typical. Another way to minimize speed error is to compare the relative speeds of two different signals.
Another type of propagation error occurs when the GPS signal is reflected from various local obstructions before it gets to the GPS receiver. This is called multipath error, which interferes with the GPS signal in a manner similar to the ghosting interference that occurs in television reception. Signal rejection techniques may be utilized to minimize this problem.
The atomic clocks in the GPS satellites are extremely precise but they are not perfect. Minute discrepancies can occur, and these translate into travel time measurement errors, and even though the positions of the satellites are constantly monitored, slight position or “ephemeris” errors may occur between monitoring times.
Another source of error is that the geometry of the GPS satellites relative to the GPS receiver can magnify the time measurement errors with a principle called “Geometric Dilution of Precision” or GDOP. If the GPS receiver selects satellites that are close together in the sky, the intersecting circles that define the GPS receiver's position will cross one another at very shallow angles, increasing the ambiguity of the point of intersection. One the other hand, if the GPS receiver selects satellites that are widely separated, then the circles intersect at almost right angles, which minimizes the ambiguity.
Differential GPS, or DGPS, can yield measurements accurate to a couple meters in moving applications and even better in stationary situations. In differential GPS, a stationary GPS receiver is required in addition to the GPS receiver that is measuring its position. Because CDMA communications systems already include a stationary base station, differential GPS may be readily implemented for each mobile station. As described above, GPS receivers use timing signals from typically four satellites to determine a position. Because each of the four timing signals is subject to some error, the position calculation results in a compounding of the errors. A method of correcting these errors is to include an additional stationary GPS receiver in the base station as a reference. If the base station and the mobile station are fairly close to each other, say within a few hundred kilometers, the signals that reach both of them from the same satellite will have traveled through virtually the same slice of atmosphere, and therefore will have almost identical errors. The base station receives the same GPS signals as the mobile station, however, instead of using timing signals to calculate its position, the base station uses its known position to calculate what the travel time should be and compares the predicted travel time with the measured travel time. The difference between the predicted travel time and the measured travel time is transmitted to the mobile station. The mobile station applies the corrections to its travel time measurements and calculates its position from the corrected time measurements. The base station may predict from a previously known position of the mobile station which of the many available satellites the mobile station should use to calculate its position and only send the errors for those specific satellites to the mobile station, or the reference receiver may compute the errors for all of the available satellites and transmit all the error information to the mobile station. Alternatively, the mobile station may transmit the travel time measurements to the base station so that the base station can apply the corrections and calculate the position of the mobile station.
Even further accuracy may be obtained for GPS measurements by using the GPS carrier frequency. As explained above, a GPS receiver determines the travel time of a signal from a satellite by comparing a pseudo-random code with an identical code in the signal from the satellite. The GPS receiver delays its copy of the pseudo-random code later and later in time until it matches up with the satellite signal. The amount of delay applied to the copy is equal to the signal travel time from the satellite. The bits (or cycles) of the pseudo-random code are about a microsecond wide, however, so wide that even if the codes are matched, there is still the possibility of a 300 meter error. Code-phase GPS receivers can reduce the phase error to one or two percent, or 3–6 meters of error. Carrier-phase GPS receivers use both the pseudo-random code delay information and then perform measurements on the carrier frequency that is modulated by that code. The carrier frequency is much higher than the bits (or cycles) of the pseudo-random code, typically about 1.57 Ghz, which provides more accurate phase matching. Using a technique similar to code-phase receivers to achieve an accuracy of one or two percent phase matching, an accuracy of 3 or 4 millimeters may be realized for surveying, scientific, and other demanding applications. In essence this technique counts the exact number of carrier cycles between the satellite and the receiver. If the pseudo-random code measurement can be made accurate to say, a couple meters, then only a few wavelengths of the carrier signal have to be counted to determine which cycle actually marks the starting edge of the timing signal.
GPS used in conjunction with communication links such as CDMA communications systems can provide the backbone for systems tailored to applications in agriculture, mass transit, urban delivery, public safety, and vessel and vehicle tracking.
A modern development in cellular telephone technology is the incorporation of global positioning system (GPS) circuitry in a code division multiple access (CDMA) chip set for mobile stations so that the same radio frequency (RF) components in mobile stations may be used for both CDMA service and global positioning system functions. The global positioning system function has many applications, including supporting emergency 911 calls and other location services. Because the same radio frequency (RF) components are used for both CDMA service and Global Positioning System functions on different frequency channels, the mobile station cannot receive and respond to transmissions from the base station during the time the global positioning system function is being used to measure the mobile station location.